1. Field of the Invention
The present invention relates to an azimuth/attitude detecting sensor according to a strap-down system, which detects an azimuth/attitude of a mobile unit such as a ship or car.
2. Description of the Related Art
Conventionally, an azimuth/attitude detecting sensor according to a strap-down system includes X, Y and Z accelerometers which detect accelerations in respective axial directions and X, Y and Z gyros which detect angular velocities around respective axes (see Japanese Patent No. 3038452).
This azimuth/attitude detecting sensor uses gyro signals obtained from the gyros to calculate a matrix for transforming a coordinate system of a body to which a sensor is attached (called a “body coordinate system”) to a coordinate system in which the X-axis direction corresponds to the north direction and the Z-axis direction coincides with the gravity direction (called a “local coordinate system”), calculates an azimuth, pitch angle and rolling angle which are angles of rotation around the respective axes with respect to the local coordinate system from this matrix, outputs an azimuth/attitude and corrects attitude errors caused by gyro errors through gravity acceleration detection detected by the accelerometers.
However, the accelerometers also detect motion acceleration other than gravity, and therefore a problem is that an attitude error occurs under the influence of motion acceleration. Using costly and high-performance gyros and accelerometers allow an azimuth/attitude detecting sensor less susceptible to motion acceleration to be constructed, but the influence of the motion acceleration when a low cost, low precision gyro or accelerometer is used is considerable and a problem is that they are unusable for practical applications depending on the motion state.
As a structure for eliminating such influences of motion acceleration, for example, an X velocimeter is provided to detect a velocity in the X-axis direction for correcting acceleration errors is known from Japanese Patent Laid-Open No. 6-201863.
When a mobile unit performs turning motion, the mobile unit which cannot bear a centrifugal force occured during the turning motion, produces a side slipping which causes the velocity in the traveling direction not to match the actual trajectory velocity. The side slipping produces an error of the angles of attitude because only the velocity in the X-axis direction can be detected and an acceleration component due to the velocity in the Y-axis direction cannot be compensated. Japanese Patent Laid-Open No. 6-201863 proposes to determine an angle of the side slipping from an azimuth signal and velocity signal in the X-axis direction through a calculation in order to prevent the error of the angles of attitude produced by the side slipping.
However, such a calculation described in Japanese Patent Laid-Open No. 6-201863 requires a velocimeter and also requires a calculation apparatus calculating the angle of side slipping, resulting in a problem that the size and cost of the apparatus increase.
Furthermore, with regard to azimuth compensation, it is conventionally a general practice to use an azimuth signal detected by a magnetic azimuth sensor from Japanese Patent No. 3038452 or Japanese Patent Laid-Open No. 11-94573. For example, Japanese Patent Laid-Open No. 11-94573 discloses a structure in which when a traveling azimuth detected by a gyro is compensated, an azimuth detected by a geomagnetism sensor is assumed to be a traveling azimuth and the azimuth detected by the gyro is followed to the azimuth detected by the geomagnetism sensor.
However, according to the structure in Japanese Patent Laid-Open No. 11-94573, when the magnetic azimuth sensor detects a wrong azimuth under the influence of surrounding magnetic field environment, the wrong direction is assumed to be the traveling azimuth. Moreover, since the azimuth detected by the geomagnetism sensor is unconditionally assumed to be the traveling azimuth, a problem is that in the case of a motion state in which the traveling direction differs from the axial direction of the magnetic azimuth sensor, a wrong direction may be assumed to be the traveling direction.
Taking the problem of such a geomagnetism sensor into consideration, Japanese Patent Laid-Open No. 2002-158525 proposes to use a GPS receiver.
According to Japanese Patent Laid-Open No. 2002-158525, a most reliable estimated value is selected as an absolute azimuth reference value according to a situation of the mobile unit at the present moment, an absolute azimuth detected by a magnetic compass is regarded as an absolute azimuth reference value when a certain condition is satisfied and a traveling direction azimuth detected by a GPS receiver is regarded as an absolute azimuth reference value when another condition is satisfied.
However, in Japanese Patent Laid-Open No. 2002-158525, when the reference value changes from the absolute azimuth detected by the magnetic compass to the traveling direction azimuth detected by the GPS receiver depending on change of the condition, which produces a problem that the reference value changes drastically and it lacks reliability as a reference value.
The present invention has been implemented in view of such problems and it is an object of the present invention to provide an azimuth/attitude detecting sensor capable of making highly reliable azimuth error compensations using a GPS direction detected by GPS receiver and magnetic azimuth detected using a magnetic azimuth sensor.
Furthermore, it is another object of the present invention to provide an azimuth/attitude detecting sensor when making level error compensations using acceleration detected by an accelerometer, capable of making highly reliable level error compensations in a simple structure and at low cost.